The invention relates to a method of and apparatus for the separation of the components of gas mixtures by liquefaction, and can be applied in various areas of technology, including application to liquefaction of a gas, for example for use in gas and petroleum processing including, metallurgy, chemistry and other areas of technology.
A widely used method for the liquefaction of gas indudes compression of gas in a compressor, preliminary cooling in a heat exchanger and further cooling in an expander with subsequent expansion of the gas through a throttle valve to cause cooling and condensation. Subsequently the liquid phase is selected and separated (see Polytechnic Dictionary, 1989, Moscow, xe2x80x9cSovetskaya Entsiklopediyaxe2x80x9d, p. 477, Ref. 1). A disadvantage of this known method is the implementation complexity in operation, and sensitivity to liquid drops in the inlet gas flow.
A known method for the separation of the components of gas mixtures by means of liquefaction includes cooling of the gas mixture in stages to the condensation temperature of each of the components and the separation of the corresponding liquid phase at each stage (see Japanese patent application No. 07253272, F 25 J 3/06, 1995, Ref. 2). A disadvantage of this known method is its small efficiency while requiring a large amount of energy.
Another known method for the separation of the components of gas mixtures by means of their liquefaction includes adiabatic cooling of the gas mixture in a supersonic nozzle and the separation of the liquid phase (see U.S. Pat. No. 3,528,217, U.S. Cl. 55-15, Int. Cl. V 01 D 51/08, 1970, Ref. 3). In this known method, the separation of the liquid phase is performed by passing the gas-liquid mixture around a perforated barrier by deflection of the flow from a simple linear flow. As a result, centrifugal forces arise due to the deflection of the flow, and under the action of these centrifugal effects, drops of liquid are displaced radially outwards. The liquid drops then pass through the perforated barrier, so as to be separated, and are collected in a container. A disadvantage of this known method is its low efficiency. The reason for this low efficiency is that under the deflection of the gas flow that moves with supersonic speed, shock waves occur, which raise the temperature of the gas, and this leads to the unwanted vaporization of part of condensed drops back into the gaseous phase.
Among the known methods, a method that is the closest to the present invention consists of the separation of gas components by their liquefaction (as disclosed in U.S. Pat. No. 5,306,330, U.S. Cl. 95-29, Int. Cl. V 01 D 51/08, 1994, Ref. 4). This known method can be used to separate the components of a gas mixture. (See column 1, lines 5-10, Ref. 4).
The method in Ref. 4 includes cooling of a gas in a supersonic nozzle and the separation of the liquid phase. A shock wave is present at the nozzle, and the invention relies on droplets, already formed, having a greater inertia. Hence, the droplets maintain a higher velocity downstream, facilitating their separation by centrifugal effects. To separate the liquid phase, the cooled gas flow, which contains already drops of a condensed liquid phase, is deflected through a curve, away from the initial axis of the nozzle. As a result of the deflection of the flow, under the action of the inertia, and centrifugal forces, the droplets with a higher velocity are displaced radially outwards from the axis of the flow. The flow is then divided into two channels, and one portion of the flow containing the droplets is passed along one channel, and another portion of gas flow, substantially dry and free of liquid droplets, passes along another channel. This technique bears some similarities with Ref. 3, in that the gas is effectively rotated or caused to turn about an axis perpendicular to the original axis and flow direction of the nozzle.
A disadvantage of this known method is its low efficiency. This is due to the fact that under such a deflection of the gas flow, shock waves again occur, and thus the temperature of the flow increases, which leads to the unwanted evaporation of part of the condensed droplets.
Moreover, when liquefying a selected component, the partial pressure of the remaining gas phase is reduced. Hence, for a more complete (subsequent) liquefaction, one must provide for a decrease of the static temperature of the flow. This can be achieved by means of an increase of the rate of the adiabatic expansion of the flow, and hence by the corresponding increase of its Mach number. This requires a substantial reduction of the output pressure of the flow, which drastically reduces the efficiency of this technology, in terms of power requirements.
There is yet another known device for the separation of the components of gas mixtures and isotopes that contains an evaporator, a curvilinear supersonic nozzle, a separator in the form of a cooled knife, and receivers for the separated components (see the description to the patent pending of Russian Federation No. 2085267, V 01 D 59/18,1997, Ref. 5). Disadvantages of this known device are the complexity of the construction and low efficiency with respect to both the energy efficiency of the process and to the extent of the separation.
All the above methods of Ref. 2-5 have a common disadvantage that significantly reduces their efficiency and that results from the existence of a shock wave due to the change of the gas flow direction. These shock waves both heat the gas, leading to vaporization of the drops, and significantly decrease the total head at the outlet of the apparatus.
The present invention is intended to improve the efficiency of the separation of gas mixtures by means of their liquefaction and of the liquefaction of a gas, and is intended to provide separation of gas components at the instant of liquefaction.
This desired result is accomplished, in the present invention, by the provision of a method for the liquefaction, which includes adiabatic cooling of a gas mixture or a gas in a supersonic or subsonic nozzle and the separation of the liquid phase. Moreover, the present invention modifies the partial pressure of the gas or each component in the mixture. Then, in one aspect of the invention, the partial pressures in the initial mixture can be modified in the device so as to provide a higher temperature of condensation of one component, that has a lower temperature of condensation at atmospheric pressure than the temperature of condensation of another component with a higher temperature of condensation at atmospheric pressure. The geometry of the nozzle is chosen to preserve in the gaseous phase, in the course of cooling, the other component with the higher temperature of a condensation at atmospheric pressure and the liquefaction of the one component that has a lower temperature of a condensation at atmospheric pressure is in an amount that is sufficient to dissolve in it the gaseous phase of the bulk of the component that has a higher temperature of condensation at atmospheric pressure.
In accordance with a first aspect of the present invention, there is provided a method of liquefying a gas, the method comprising the steps of:
(1) applying a swirl velocity to the gas;
(2) passing the gas, with the swirl velocity, through an expansion nozzle;
(3) permitting the gas flow to expand adiabatically downstream from a nozzle in a working section having a wall, whereby the gas cools and at least a portion of the gas flow condenses to form droplets;
(4) permitting centrifugal effects generated by the swirl velocity to drive the droplets towards the wall of the working section; and
(5) separating condensed liquid gas droplets from remaining gas in the gaseous state at least adjacent the wall of the working section.
Preferably, the method includes separating condensed liquid from the gas flow downstream from the nozzle at a location spaced a distance L from the dew point, where L=Vxcfx84, where V is the speed of the gas flow at the outlet of the nozzle and xcfx84 is the time taken for condensed droplets of gas to travel from the axis of the nozzle to a wall of the working section. By the dew point we mean the zone inside the nozzle in which the change from the gas phase into the liquid phase starts.
The condensed droplets can be separated by any suitable means, for example through an annular slot or through perforations.
The method can be applied to a gas comprising a plurality of separate gaseous components having different properties, and the method further comprising adiabatically expanding the gas such that at least two gaseous components commence condensation at different axial locations downstream from the nozzle throat, to form the droplets and separating out the droplets of these gaseous components independently from each other gaseous component.
In such a case, there is then provided a separation device for each component at a location which is a distance Li from the axial location at which a corresponding gaseous component condenses, where Li is determined by the relationship Li=Vixc3x97xcfx84i; where Li is the distance between the dew point of the ith gas component to a location at which the ith gaseous component is separated; Vi is the speed of the gas flow at the dew point of the ith gaseous component and xcfx84i is the time for droplets of the ith gaseous component to travel from the axis of the nozzle to the wall of the working section.
For some gases, it may be sufficient to generate subsonic velocities, but in general it is expected that it will be necessary to generate a substantially sonic velocity in the gas close to the nozzle throat, so as to cause the gas to expand supersonically in the nozzle and in the working section.
Another aspect of the present invention provides an apparatus for liquefying a gas, the apparatus comprising:
(1) means for imparting a swirl component of velocity to a gaseous flow; and
(2) downstream from said swirl generation means, a nozzle comprising a convergent nozzle portion connected to the swirl generation means and a nozzle throat and a divergent nozzle portion (and optionally, particularly in the case of a supersonic nozzle), and a working section, whereby in use, the gas adiabatically expands in the nozzle and in the working section, to cause condensation of at least some of the gas, thereby generating droplets of condensed gas.
In a particular aspect of the present invention, it is applied to a gas having a plurality of gaseous components in the mixture; and the partial pressures of these components is such that, when the gas flow passes through the nozzle, one component, that has a lower temperature of condensation at atmospheric pressure than the temperature of condensation of another component, has a partial pressure such as to cause it to condense first during adiabatic expansion. For example, for natural gas a high partial pressure for methane can cause it to condense first in an amount sufficient to dissolve the ethane, still in the gaseous state.
For this aspect of the invention, a geometry of the nozzle is selected so as to ensure the preservation in the gaseous phase, in the course of cooling, of the component with the higher temperature of condensation at atmospheric pressure; more particularly, the geometry of the nozzle is chosen to ensure the condensation of the component that has a lower temperature of condensation (at atmospheric pressure) in a quantity sufficient to dissolve in it the bulk of the gaseous phase of the component that has a higher temperature of condensation.
This permits one to increase the efficiency of the separation of gas fractions for the following reason. In the gas flow, the gas component that has a lower temperature of condensation at atmospheric pressure is then the first component that starts to condense. This leads to the appearance of a lot of small drops (a fog), which dissolve in themselves the bulk of the component that has a higher temperature of condensation (at atmospheric pressure) and thus removes the latter component from the mixture.
This also permits one to increase the efficiency of the separation of gas fractions in a mixture because the gas component that has a higher temperature of condensation, which is preserved in the gaseous phase in the course of the adiabatic cooling, is almost completely removed from the mixture by dissolving it in the liquid phase of the other component, which is separated therefrom in a known way. Correspondingly, to remove the component that is in the gaseous phase, a sufficient amount of the other component (in the liquid phase) is needed to ensure the dissolving in it of the gaseous component.
The geometry of the nozzle that ensures the above conditions is chosen on the basis of the known laws of thermodynamics of gas and the known initial data of the gas flow, namely, the pressure at the entrance to the nozzle, the temperature of gas, the chemical composition of the mixture and the initial relation among the partial pressures, and also on the basis of reference data on the solubility of gaseous components in liquids and liquefied gases under various temperatures and pressures known at the technological level (for instance, see xe2x80x9cA Handbook on the Separation of Gas Mixtures by the Method of Deep Coolingxe2x80x9d, I. I. Gal""perin, G. M. Zelikson, and L. L. Rappoport, Gos. Nauchn.-Tekhn. Izdat. Khim. Lit., Moscow, 1963).
It is preferred for the nozzle and swirling flow to be designed to produce an acceleration of around and above 10,000 g (approximately 105 m/sec2). This acceleration is calculated on the basis that the swirling gas can be treated as a rotating solid body, i.e. the angular rotation is constant from the axis to the boundary of the nozzle. It will be appreciated, that this is a theoretical ideal model; a close approximation to this model can be achieved as a result of high swirling velocity gradients that lead to large viscosity forces.
Consequently, the actual rate of acceleration will be determined by the known formula xcfx892r, where xcfx89 is the angular velocity and r is the radius. In other words, the rate of acceleration will vary in direct proportion to the radius.
The figure of 10,000 g relates to the acceleration at the outer edge of the swirled flow, i.e. adjacent the nozzle wall. It can be achieved, with r=0.1 m and xcfx89=1,000 secxe2x88x921.
It can also be noted that instead of an exact acceleration figure, the acceleration can be defined in functional terms. Thus, the key requirement is that the losses due to friction should not be too high, i.e. the angular velocity should not be too great, and at the other extreme, drops of a diameter less than 5 microns should be caused to travel to the wall of the working section within a reasonable length. Additionally, the pressure drop should be competitive with other techniques.
At the end of working section a device for the separation of liquid (in mixture with the part of gas flow directed in the boundary layer) is provided.
The liquid withdrawal device can be adjacent a supersonic diffuser; moreover, the liquid withdrawal device and the supersonic diffuser can be essentially integral with one another. The supersonic diffuser provides for the partial transformation of the gas flow kinetic energy to an increased pressure. Thus, the liquid withdrawal device can include an edge or lip in the working section which simultaneously forms a leading edge of the supersonic diffuser channel. Such a configuration is chosen in order to increase the efficiency of the supersonic diffuser, strongly, of the order of 1.2 to 1.3 times, as compared to a standard construction of the supersonic diffuser.
Downstream from the supersonic diffuser, a subsonic diffuser is preferably provided, which both provides for further recovery of the axial kinetic energy and may include a device for recovery of the rotational kinetic energy, so as to remove the swirl component of the flow. The location of this device is in a zone where the Mach number M is 0.2-0.3, so as to give the best efficiency.